What you are proposing is not only feasable but I am doing it, though I am not using knoppix in my bridge I am using OpenBSD, but the configuration should roughly be the same. What you are doing is turning a$700 computer into a $5.00 bridge.

in the /etc just configure the interfaces:

not as dhcp
not to a static ip but as "up"

example


hostnamexl0
#dhchp
#inet
up

then
hostnamexl11
#dhcp
#inet
up

you can also add packet filtering or have the bridge do NAT

since you want to use knoppix though and knoppix is debian based I reccomend you read the following: http://www.wlug.org.nz/WirelessEthernetBridge

take care and good luck,